Fibre volume fraction - A260
| Fibre volume fraction | |
|---|---|
| Foundational knowledge article | |
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| Document Type | Article |
| Document Identifier | 260 |
| Themes | |
| Relevant Class |
Material |
| Tags | |
| Prerequisites |
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Introduction[edit | edit source]
Fibre volume fraction (Vf) is a critical composite property that describes the ratio of the volume of fibres (vf) to the total volume of a composite (v), which includes the fibre, matrix, voids, fillers, etc.
Scope[edit | edit source]
This page defines fibre volume fraction, explains its significance in both composites processing and final part performance, and the parameters that influence its value. Methods for physical measurement are briefly discussed.
Significance[edit | edit source]
In composites processing, the structural metric of fibre volume fraction is involved in several mechanical and manufacturing behaviour topics. Examples include the overall stiffness of a cured laminate and the resin permeability of a dry preform.
Prerequisites[edit | edit source]
Recommended documents to review before, or in parallel with this document:
Definition[edit | edit source]
Fibre volume fraction (Vf) describes the ratio of the volume of fibres (vf) to the total volume of a composite (v), which includes the fibre, matrix, and voids. It is calculated as follows:
\(V_f = \frac{v_f}{v} = 1 - V_m - V_v\)
Where,
- vf = Volume of fibres
- v = Total composite volume (volume matrix + volume fibres + volume voids)
- Vm = Volume fraction matrix
- Vv = Volume fraction voids
Fibre volume fraction is reported as a decimal fraction or a percentage value. Material datasheets often report the composite constituent amounts in weight fractions instead of volume fractions, and confusion between volume and weight fraction should not be made. More information can be found here: [A213]. Conversions between weight and volume can be made if both matrix density and fibre density are known as shown below.
\(W_f = \frac{V_f \rho_f}{V_f \rho_f + (1 - V_f - V_v) \rho_m}\)
Where,
- Wf = Fibre weight fraction
- ρf = Fibre density
- ρm = Matrix density
Impact on mechanical properties[edit | edit source]
Fibre volume fraction is one of the primary governing parameters for the mechanical properties of composites, such as strength, stiffness, and fatigue resistance[1]. A higher fibre volume fraction typically results in improved mechanical properties. As such, fibre volume fraction is typically of greater interest to the composite part designer, whereas fibre weight fraction is more commonly reported by material fabricators on technical datasheets [A213].
Factors controlling achievable fibre volume fraction[edit | edit source]
The ability to achieve a high fibre volume fraction depends heavily on both the chosen manufacturing process and material properties. For example, the spray-up process typically results in a lower fibre volume fraction compared to the filament winding process[2]. Because the distinct geometric arrangements of various fibre architectures dictate how tightly individual fibres can physically pack together, the choice of architecture directly impacts the resulting fibre volume fraction.
Manufacturing steps such as debulking and consolidation apply external pressure to compress the fibre architecture, remove entrapped air, and squeeze out excess resin, ultimately increasing the fibre volume fraction [A158], [P171].
| Manufacturing technique | Fibre volume fraction range (%) | ||
|---|---|---|---|
| Random mat | Woven roving | Unidirectional and multiaxial | |
| Spray-up | 10-20 | ... | ... |
| Hand lay-up | 10-20 | 25-40 | 40-50 |
| Vacuum infusion | 20-30 | 40-50 | 50-65 |
| Resin transfer moulding | 20-30 | 40-50 | 50-65 |
| Prepreg compression moulding | ... | 40-55 | 50-70 |
| Filament winding | ... | ... | 50-70 |
| Pultrusion | 20-30 | 40-55 | 50-70 |
Prepregs and resin infusion based processes[edit | edit source]
Prepreg-based processes provide a high degree of control of the fibre volume fraction because the resin-to-fibre ratio is strictly predetermined by the material supplier. Liquid resin infusion processes generally yield lower fibre volume fractions because of the lowered permeability of highly compacted fibres [A158]. Also, highly permeable flow media can remain in the part, creating non-structural resin-rich zones, and excess resin is lost to vacuum lines and other consumables [P171].
The rule of mixtures[edit | edit source]
The rule of mixtures is a fundamental mathematical principle that uses the fibre volume fraction of a composite to estimate the mechanical properties of a single laminate ply[1]. For more details, refer to Micro-mechanics - A245.
Idealized packing[edit | edit source]
Theoretically, perfectly close-packed cylinders, representing fibres, can achieve a fibre volume fraction of over 90%[2]. In reality, this is unattainable, partly because extensive fibre-to-fibre contact would create significant structural and manufacturing issues. Direct contact prevents the resin from fully enveloping the fibres, which hinders effective load transfer and introduces stress concentrations that can lead to premature fibre breakage. Extreme compaction reduces the preform's permeability making it more difficult for resin to flow and completely wet out the reinforcement during infusion processes, increasing the risk of dry spots and high void content [A158]. However, high fibre volume fractions of 60–70% are still achievable in practice.
Three idealized configurations of circular fibre packing are identified in the figure below. In open-packing, fibres are arranged in concentric layers; in square-packing, they form a square lattice; and in close-packing, they are arranged in a dense hexagonal pattern.
Measuring fibre volume fraction[edit | edit source]
The fibre volume fraction of a composite material can be determined using standardized experimental methods. The cured composite thickness method is a simpler, non-destructive approach suitable for flat laminates with minimal surface roughness [M109]. It calculates the reinforcement content based on the ply's known areal weight, density, and the cured laminate's physical dimensions. Alternatively, matrix removal methods, such as burn-off (ignition) or chemical acid digestion physically dissolve or burn away the polymer matrix to weigh the remaining reinforcement. For more details, refer to Measuring reinforcement content - M109.
Related pages
References
- ↑ 1.0 1.1 1.2 [Ref] Peters, S. T., ed. (1998). Handbook of Composites. Springer US. doi:10.1007/978-1-4615-6389-1. ISBN 978-0-412-54020-2.CS1 maint: date and year (link)
- ↑ 2.0 2.1 [Ref] Campbell, F.C. (2004). Manufacturing Processes for Advanced Composites. Elsevier. doi:10.1016/B978-1-85617-415-2.X5000-X. ISBN 9781856174152.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
- ↑ [Ref] Hoebergen, Arlen; Holmberg, J Anders (2001). Vacuum infusion. Vol 21.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
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